The present invention relates to an electromagnetic valve which is operated by an energization control of a solenoid and has a valve member for opening and closing a fluid passage, and more particularly, to an electromagnetic valve suitable for an injector of an electronic-controlled fuel injecting apparatus that is used in an engine using gasoline, LPG, alcohol, etc. as fuel, a control valve for use in a hydraulic apparatus such as an automatic speed control apparatus, or the like.
To begin with, one application of a conventional electromagnetic valve will now be explained.
Recently, in vehicles powered by a gasoline engine, electronic-controlled fuel injecting apparatuses are being used as a fuel feeding apparatus in order to improve fuel consumption, the output characteristic of the engine and purifying of exhausted gases. The electronic-controlled fuel injecting apparatuses use fuel injecting valves (hereinafter simply called "injectors") or an electromagnetic valve to inject fuel, which is normally kept at a constant pressure, into an intake passage located, for example, upstream or downstream of a throttle valve or located at an intake port, or into a fuel combustion chamber, and employ, as an injection rate control system, an intermittent injection system which opens the injector for a certain period of time in accordance with the amount of air supplied per engine cycle. That is, the amount of injecting fuel depends only on the width of an electric pulse applied to the injector, thus ensuring easy and highly accurate fuel control. Further, since operation control parameters for the engine are all converted into electric signals based on which the pulse applying duration is determined, the system for calculating the pulse applying duration has a greater freedom for its modification and correction parameters can easily be added.
The injector comprises a housing, a valve body mounted on the distal end of the housing, a solenoid fixedly supported in the housing, and a valve needle slidably fitted in a guide hole formed in the valve body and coupled to a plunger which is made of a magnetic material and is driven by the electromagnetic force of the solenoid in cooperation with a spring. When the electric pulse is applied to the solenoid, the plunger is attracted against the force of the spring. With the movement of the plunger, the valve needle is pulled to inject fuel through an injection opening of the valve body. The fuel injection rate is determined by the circular gap between the distal end of the valve needle and the injection opening of the valve body and the fuel pressure.
To improve the durability of such an injector, the valve body and valve needle are usually made of stainless steel such as SUS440C having wear resistance and are subjected to heat treatment to increase their hardness.
Another use of a conventional electromagnetic valve will now be explained.
A hydraulic apparatus such as an auto-transmission control system uses many hydraulic control valves, which are usually solenoid valves that are easy to control. This type of control valve typically comprises a solenoid accommodated, in a housing, a valve guide also accommodated in the housing, and a valve rod slidably fitted in a guide hole formed in the valve guide. The valve rod is coupled to a plunger that is driven by the electromagnetic force of the solenoid in cooperation with a spring. When the solenoid is deenergized, the force of the spring pushes the valve rod to close the control valve. When the solenoid is energized, the electromagnetic force of the solenoid attracts the plunger against the force of the spring so as to pull the valve rod to open the control valve.
To improve the durability of the control valve, the valve rod is usually formed of stainless steel such as SUS304 or SUS440C, which is corrosion-resisting and wear-resisting, and is further subjected to a surface nitriding process if it is SUS304, or subjected to heat treatment for surface hardening to increase the hardness of the valve rod if it is SUS440C.
As the stainless steel such as SUS304 is a ferromagnetic material, however, it would be magnetized through the manufacturing processes and iron powder would adhere to the steel. The adhesion of iron powder adversely influences the process accuracy, fluid-tight testing, flow rate setting, or the like. The stainless steel, when subjected to heat treatment, has a martensite structure and is magnetized. Further, when driven, the valve needle or valve rod (these are called "valve members") is influenced by the magnetic field of the solenoid and is magnetized. As, a result, magnetic powder produced by wearing of the sliding section between the valve needle and valve body or the valve rod and valve guide, which is caused by operation of the valve needle or valve rod, iron powder adhering to and remaining in a fuel system during the manufacturing process, or iron powder mixed in oil adheres to the sliding section, or the iron powder adhere to the sliding section falls there off and adheres the injection hole or the valve seat. Consequently, the actuation of the valve members would be interfered, thus deteriorating the fuel injection characteristic or hydraulic control characteristic, or the wearing of the sliding surface would be hastened due to the magnetic powder and iron powder coming into the sliding section. When the valve members are magnetized, attraction or repulsion is caused between the valve needle and valve body or the valve rod and valve guide, thereby interfering smooth actuation of the valve members or impairing the control characteristic.